In general, in order to increase the compression ratio, a four-stroke internal combustion engine is designed to have a combustion chamber made as small as possible in volume in a state where a piston is at a top dead center by forming a piston crown into such a bulged shape as to match a ceiling surface of the combustion chamber; and providing the piston crown with valve recesses that are recesses for avoiding interference with valves.
There is an example of a four-valve four-stroke internal combustion engine in which a piston crown is bulged into a pent-roof shape, and paired intake valve recesses and paired exhaust valve recesses are formed on the piston crown (see Japanese Patent Application Publication No. 2009-121324).
The four-stroke internal combustion engine disclosed in Japanese Patent Application Publication No. 2009-121324 is mounted on a motorcycle with a cylinder disposed in a posture of standing substantially upward. A pair of right and left intake ports are opened on a front side of a pent-roof ceiling surface of a combustion chamber, and extend obliquely frontward; meanwhile, a pair of right and left exhaust ports are opened on a rear side of the ceiling surface, and extend rearward.
The piston crown is bulged into such a pent-roof shape so as to match the ceiling surface of the combustion chamber. The piston crown is provided with a pair of right and left intake valve recesses for avoiding interference with a pair of intake valves configured to open and close intake inlets of the pair of intake ports on the ceiling surface; and a pair of right and left exhaust valve recesses for avoiding interference with a pair of exhaust valves configured to open and close exhaust outlets of the pair of exhaust ports.
Intake-side inclined flat surfaces formed by bottom surfaces of the pair of intake valve recesses and exhaust-side inclined flat surfaces formed by bottom surfaces of the pair of exhaust valve recesses intersect each other such that a pent-roof shape matches the ceiling surface of the combustion chamber.